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TE told me you changed some strings in the language files and put some new ones into them. So it seems to be my used and optimized language files are somehow incomplete.

What I need now is a simple and effective way to find those strings. Do you think it's a good idea to compare your english files with my german files with tools like WinMerge? Then every line will be marked as different because of the english and german texts... Not easy to find missing lines then. Any thoughts?

hmm...
It all depends from "where" the language files you worked on come from.
Did you grab them from the download here?

as far as I can see, for now there was only few removed keys, nothing new, but I may be wrong

We have been doing a search for unused strings and removing them.  Wish we could have done that sooner to cut the translation work, but better late then never.  Actually a bunch of them had been unused to some time, even found a few dating to before SMF forked from Yabba so some 10 year old "junk in the trunk"

For what has changed, if you have the original English strings you could do a winmerge on them vs the current repo and find what changed in English, and then go to those strings in the German files.  Should not be to many, and really you are wanting what may have been added, the changed ones are probably not that important (for the most part, it was just phrasing vs wrong information).  Removed strings are super easy to update, and if left its harmless